In mammals, including humans, males and females differ notonly in physical appearance but in every cell of their body:male cells have a tiny Y-chromosome which females lack. Femalesinstead have two X-chromosomes whilemales have onlyone. It is not universally true though, as majority of fishes,frogs, lizards and turtles have no sex chromosomes. Their sexis generally determined based on the environment (e.g. temperature)in which the eggs grow. Though, the Y-chromosomegene, Sry, triggers male development, it alone is not enoughto differentiate the two sexes; orderly expression of a numberof genes, generally present on the autosomes, is requiredto ensure differentiation of a specific gonad – testis or ovary.Individuals bearing testes become male while those havingovaries become female. Excepting the Y-chromosomal Sry,almost all other genes in this cascade are evolutionarily conservedthroughout vertebrates. Mutually antagonistic interactionsof the male and female pathway genes lead to the formationof the gonads that eventually determine the sex of theindividual. Most disorders of sexual development occur dueto mutations in any of these genes.